Hiss Reduction in old audio recordings
There are lots of audio restoration techniques available with programmes like "Cooledit" or "Cooledit Pro".
This short article gives a method of audio cleanup which will remove most of the hiss from an old recording
without damaging the quality of the sound underneath.
The program I'm describing is Cooledit Pro, but other packages can be persuaded to do a similar job.
There are three methods available:
1. Use the Cooledit hiss reduction tool as recommended.
2. Use the noise reduction tool, sample the hiss and then subtract it from the original recording.
3. The following method, which gives an improved signal to noise ratio over either of the above.
So - here's the method. It won't make you into an audio expert - that needs endless hours of practice -
but you may be surprised by how well it works. (Methods 1 and 2 will sometimes work quite well but they
can make the recording "tinny" because of the way the noise reduction algorithm works).
What you do is to sample the hiss, then invert it and add it back to the
original signal so that it (largely) cancels out.
Here's the sequence of operations:
Load up Cooledit Pro; load your file...
1. Now use the hiss reduction tool (low, standard or high) on a short
piece of the recording - just a few seconds to begin with.
2. Check "keep only hiss". This copies the hiss to the clipboard. Most of the signal will disappear from
the screen.
3. Use EDIT - COPY, which saves the hiss to the clipboard.
4. Undo the hiss reduction; your original file returns to the screen.
5. Use EDIT - MIX PASTE
6. Tick "lock left/right" and "invert" and "from clipboard" and "overlap mix", then "OK".
This adds the inverted noise back to the original recording.
7. Then save the file under another name.
Compare it with the original. It's interesting that this is much better than
using the hiss reducer in the normal way.
Hiss reduction is just one stage of audio cleanup which makes an old recording more listenable.
Nigel Deacon / Diversity website
